Electric heater



March 17, 1936. ,1 g SHARP 2 ELECTRIC HEATER Filed March 10, 1934 Inyentov:

John CSharp y l9 i Hi5 Att orney.

Patented Mar. 17, 1936 UNITED STATES PATENT OFFICE I ELECTRIC HEATER Application March 10, 1934, Serial No. 714,982

7 Claims.

This invention relates to electric heaters, more particularly to electric heaters of the sheathed type, and it has for its object the provision of an improved heater of this character which can be operated successfully at materially higher tem-- peratures and which has a considerably longer life than the heaters heretofore known.

The invention has particular application to electric heaters of the sheathed type, such as de- 10 scribed and claimed in the United States patent to C. C. Abbott, No. 1,367,341, dated February 1, 1921. Briefly, heaters of this type comprise a resistance heating element enclosed in a metallic sheath, and embedded in and supported in spaced relation with respect to the sheath by a densely packed layer of heat refractory and heat conducting, electrically insulating material, such as magnesium oxide. It is to be understood, however, that this invention is not limited to the specific heater disclosed in this patent, but has general application.

The metallic sheaths of the heaters heretofore generally used have been formed of various materials, such as steel, calorized steel, etc. An-

other sheath is formed of a nickel-chromium alloy having a chromium content of approximately 18% and a nickel content of approximately 8%, the remainder consisting chiefly of iron.

However, none of the heaters provided with these sheaths could be operated at the high temperatures contemplated by this invention. The steel sheaths rapidly oxidized at the higher temperatures.

The above-mentioned nickel-chromium alloy sheath is fairly satisfactory for operating temperatures up to about 1400 F., but for many applications should not be operated at temperatures much above 1000 F. This sheath has a critical operating temperature between 1000- 1400" F., and when operated at or above this range, rapidly weakens and disintegrates. Intermittent heating, such as encountered in domestic range service, hastens the failure of this alloy sheath. It is not known exactly why the sheath deteriorates when operated at these higher temperatures, but there is good evidence to show that its failure is due to carbide precipitation in the 50 sheath in the form of chromium carbide. This precipitation occurs even when the sheath is operated at temperatures lower than the critical temperature range mentioned above, but it is accelerated materially b'y alternate heating and 55 cooling through the higher temperature range.

Chromium carbide precipitation, it is believed, impoverishes the chromium in the grain boundaries and causes intergranular failure along these boundaries due to oxidation of the ferric phase that is formed. It has been found that eventually the grains separate to such an extent as to definitely weaken the mechanical structure of the sheath.

In addition to this, it has been found that when heaters provided with this type of sheath are 10 operated at high temperatures there is during the life of the heater a marked decrease both in the electrical insulation resistance of the refractory insulating material in the sheath of the heater and also in its dielectric strength. Here 15 again, it is not known exactly why this sheath causes these effects.

This invention contemplates the provision of an improved electric heater which can be operated successfully at temperatures of 1400-1600 20 F., or even higher values, for relatively long periods of time without failure either of the sheath or of the electrical insulation, and which has improved dielectric characteristics.

In accordance with this invention, the electric 25 heater is provided with a metallic sheath which is formed of an alloy consisting mainly of nickel and containing a substantial proportion of chromium. It is now believed that the most satisfactory results Will be obtained if the sheath be 3 made of an alloy having a nickel content of 30-80% and a chromium content of 10-20%. One alloy which has been quite good consists of approximately 14% chromium, 6% iron and the remainder consisting chiefly of nickel with minor 35 quantities of carbon, manganese, silicon, phosphorus, sulphur and copper.

For a more complete understanding of this invention, reference should be had to the accompanying drawing in which the single figure is a 40 fragmentary View in elevation of an electric heater embodying this invention, portions of the heater being shown in section so as to illustrate certain structural details.

Referring to the drawing, this invention has 45 been shown in one form as applied to an electric heater of the type described and claimed in the above-mentioned Abbott Patent No. 1,367,341. Briefly, this heater comprises a helical resistance conductor l0 mounted within and centrally of a tubular metallic sheath I l. The conductor I0 preferably will be formed of a nickel-chromium alloy having a nickel content of approximately 80% and a chromium content of about 20%. The resistance conductor I0 is embedded in and supported in spaced relation with respect to the sheath I I by a densely compacted layer of a suitable heat conducting, heat refractory electrically insulating material 52, such as powdered magnesium oxide. The insulating material l2 may be compacted to a hard dense mass having good heat conductivity in any suitable manner,"'such as by reducing the diameter of the sheatlnafter the unit has been assembled. This may be ac-= complished by swaging or rolling. The resistance conductor to has its ends secured to termi= nals it which project from the ends of the sheath and which have their inner ends embedded in the insulating material 52 so as to be supported in spaced relation with the sheath.

The metallic sheath ii, in accordance with this invention, is made of a nickel-chromium alloy consisting mainly of nickel and containing a substantial proportion of chromium with relatively minor quantities of iron, carbon, manganese, silicon, phosphorussulphur and copper. It has been found that a very satisfactory alloy consists of approximately 14% chromium, 6% iron, .10% carbon, 1.00% manganese, 15% sillcon, .025% phosphorus, .025% sulphur, 35% copper and the remainder of nickel. It is to be understood, however, that this invention is not limited to these specific proportions. The chromium and nickel contents may vary somewhat widely. It is important, however, that the combined chromium and nickel content of the alloy be 50% or more with the chromium content ranging between 10% and 20% and the nickel content between 30% and It has been found that an electric heater made in accordance with this invention will operate without failure at very high temperatures and for very long periods of time. Actual tests have demonstrated that the metallic sheath (does not disintegrate'and decay with use as do the sheaths mentioned in a previous portion of this specification.

Moreover, it has been found that the sheath retards materially the drop in the electrical insulation resistance of the refractory insulating material it which has been experienced to a marked degree in electric heaters heretofore known.

In addition, this heater has far superior dielectric characteristics than the heaters heretothe electric heater heretofore known, even though it be operated at much higher temperatures than these prior heaters.

Briefly, the chief advantages of my heater reside in substantial freedom from decomposition of the sheath under intermittent or continuous service at relatively high temperatures, such as 1600 F. or more; maintenance of the insulation characteristic of the heater throughout the temperature range of the heater; and improved dielectric characteristics. In addition to this, my sheath material is easier to work mechanically than most of the sheath materials heretofore used, that is, it lends itself readily to the swaging or rolling operations used to reduce the diameter of the sheath to compact the insulating material i2, and the completed heater may be readily bent and shaped as desired.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not wish to be limited thereto since many modifications may be made, and I, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A sheathed electric heater capable of being operated at temperatures as high as approximately 1600 F. during a relatively long life comprising a resistance conductor embedded in an electrically insulating heat conducting mass and a metallic sheath encasing said mass formed of an alloyconsisting chiefly of nickel, chromium and iron, the combined chromium and nickel content being at least approximately 50% of said alloy.

2. A sheathed electric heater capable of being operated through a high temperature range oi approximately 1400-1600 F. during a relatively long life comprising a metallic sheath formed or an alloy consisting chiefly of nickel, chromium and iron, the combined nickel and chromium content of said alloy being at least approximately 50% of said alloy with the nickel content ranging between 30% and 80% of said alloy and the chromium content ranging between 10% and 20% of said alloy.

3. A sheathed electric heater for electric ranges and the like capable of being operated at elevated temperatures, such as approximately 1600" R, comprising a metallic sheath formed of an alloy consisting of at least 10%-20% chromium and at least 30%-80% nickel, the combined nickel and chromium content being at least 50% of said allloy, and the remainder of said alloy consisting chiefly of iron, powdered magnesium oxide compacted in said sheath and an electrical resistance element embedded in said magnesium oxide in spaced relation with said sheath, said heatermaintaining a relatively high insulating value when operated at temperatures as high as 1600 F. for relatively long periods of time.

4. An electric heater comprising a metallic sheath formed of an alloy consisting mainly of nickel, with approximately 14% chromium, 6% iron and relatively minor quantities of carbon, manganese, silicon, sulphur, phosphorus and copper, an electrical resistance element encased by said sheath and powdered heat refractory electrically insulating material embedding said resistance element and supporting it in spaced relation with said sheath.

5. An electric heater comprising a sheath formed of an alloy consisting of approximately 78% nickel, approximately 14% chromium, approximately 6% iron and minor quantities of carbon, manganese, silicon, sulphur, phosphorus and copper, an electrical resistance element encased by said sheath and powdered magnesium oxide embedding said resistance element and supporting it in spaced relation with said sheath.

6. A tubular sheath for an electric heater wherein a resistance element embedded in an electrically insulating heat conducting mass is encased by said sheath consisting of approximately 78% nickel, 14% chromium, 6% iron, and minor quantities of carbon, manganese, silicon, sulphur, phosphorus and copper.

7. A sheathed electric heater for electric ranges and the like capable of being operated at an elevated temperature of approximately 1600 F. comprising a sheath formed of an alloy consisting of approximately 14% chromium, 6% iron and the remainder chiefly oi nickel, an electrical resistance element encased by said sheath and. powdered heat refractory electrically insulating material embedding said resistance element and supporting it in spaced relation with said sheath. 

